Problem with positioning object relatively to another object
I finally found what causes this problem.
The object origin was not right for both objects.
https://docs.blender.org/manual/en/dev/editors/3dview/object/origin.html

For each object : 
Panel:  Tool Shelf ‣ Tools ‣ Edit ‣ Set Origin
Menu:   Object ‣ Transform
Hotkey: Shift-Ctrl-Alt-C
-> Origin to Geometry

However I am still curious if there is a smarter way to specifies that 2 faces should be on the same plan.